We prove the Plancherel formula for Whittaker functions on a reductive p-adic group. The methods are close to those of the proof of Waldspurger, folllowing Harish-Chandra, for smooth functions on the group. We hope also that this article suggests the right framework for studying other Plancherel formulas, especially reductive p-adic symmetric spaces. It shows in particular the role played by the B-matrices and their adjunction property.